The present invention relates to gallium nitride semiconductor light emitting devices such as semiconductor lasers and semiconductor diodes, and also to semiconductor laser light source devices, and more particularly, to a light emitting device having a multi-quantum-well structure active layer made of nitride semiconductor.
As a semiconductor material for semiconductor laser devices (LDs) and light emitting diode devices (LEDs) having emission wavelengths within a wavelength range of ultraviolet to green, gallium nitride semiconductors (GaInAlN) are used. A blue LD using such a gallium nitride semiconductor is described in, for example, Applied Physics Letters, vol. 69, No. 10, p. 1477-1479, and a sectional view of the blue LD is shown in FIG. 19. FIG. 20 is an enlarged view of part E in FIG. 19.
Referring to FIG. 19, reference numeral 101 denotes a sapphire substrate, 102 denotes a GaN buffer layer, 103 denotes an n-GaN contact layer, 104 denotes an n-In0.05Ga0.95N layer, 105 denotes an n-Al0.05Ga0.95N cladding layer, 106 denotes an n-GaN guide layer, 107 denotes a multi-quantum-well structure active layer composed of In0.2Ga0.8N quantum well layers and In0.05Ga0.95N barrier layers, 108 denotes a p-Al0.2Ga0.8N layer, 109 denotes a p-GaN guide layer, 110 denotes a p-Al0.05Ga0.95N cladding layer, 111 denotes a p-GaN contact layer, 112 denotes a p-side electrode, 113 denotes an n-side electrode, and 114 denotes a SiO2 insulating film. In this arrangement, as shown in FIG. 20, the multi-quantum-well structure active layer 107 is composed of five 3 nm thick In0.2Ga0.8N quantum well layers 117 and four 6 nm thick In0.05Ga0.95N barrier layers 118, totally nine layers, where the quantum well layers and the barrier layers are alternately formed.
Also, in Applied Physics Letters, vol. 69, No. 20, p. 3034-3036, there is described a structure that the quantum well structure active layer is composed of alternately stacked three 4 nm thick quantum well layers and two 8 nm thick barrier layers, totally five layers.
Japanese Patent Laid-Open Publication HEI 8-316528 also describes a blue LD using a gallium nitride semiconductor. This prior-art blue LD also uses a multi-quantum-well structure active layer having five or more quantum well layers, as in the case shown in FIGS. 19 and 20.
Meanwhile, a blue LED using a gallium nitride semiconductor is described in, for example, the aforementioned Japanese Patent Laid-Open Publication HEI 8-316528, and a sectional view of the blue LED is shown in FIG. 21. Referring to FIG. 21, reference numeral 121 denotes a sapphire substrate, 122 denotes a GaN buffer layer, 123 denotes an n-GaN contact layer, 124 denotes an n-Al0.3Ga0.7N second cladding layer, 125 denotes an n-In0.01Ga0.99N first cladding layer, 126 denotes a 3 nm thick In0.05Ga0.95N single-quantum-well structure active layer, 127 denotes a p-In0.01Ga0.99N first cladding layer, 128 denotes a p-Al0.3Ga0.7N second cladding layer, 129 denotes a p-GaN contact layer, 130 denotes a p-side electrode, and 131 denotes an n-side electrode. Like this, in blue LEDs using gallium nitride semiconductors, an active layer having only one quantum well layer has been used.
The conventional blue LDs and blue LED described above, however, have had the following problems.
Referring first to the blue LDs, the value of oscillation threshold current is as high as 100 mA or more and so needs to be largely reduced for practical use in information processing for optical disks or the like. Further, if the LD is used for optical disks, in order to prevent data read errors due to noise during data reading, it is necessary to inject a high-frequency current of an about 300 MHz frequency into the LD and modulate an optical output power with the same frequency. In the conventional blue LDs, however, optical output power is not modulated even if a high-frequency current is injected, causing a problem of data read errors.
Referring now to blue LEDs, which indeed have been in practical use, in order to allow blue LEDs to be used for a wider variety of applications including, for example, large color displays capable of displaying bright even at wide angles of visibility, it is desired to realize even higher brightness LEDs by improving optical output power.
Furthermore, conventional gallium nitride LEDs have a problem that as the injection current increases, the peak value of emission wavelengths largely varies. For example, in a gallium nitride blue LED, as the forward current is increased from 0.1 mA to 20 mA, the peak value of emission wavelengths shifts byas much as7 nm. This is particularly noticeable in LED devices having long emission wavelengths; for example, in a gallium nitride green LED, the peak value of emission wavelengths shifts by as much as 20 nm. When such a device is used in a color display, there would occur a problem that colors of images vary depending on the brightness of the images because of the shift of the peak wavelength.
In view of the above, a primary object of the present invention is to solve the above-described problems of the gallium nitride semiconductor light emitting devices and provide a gallium nitride semiconductor light emitting device which makes it possible to realize a semiconductor laser diode having satisfactory laser oscillation characteristics as well as a light emitting diode capable of yielding high optical output power.
A further object of the present invention is to provide a gallium nitride semiconductor light emitting device which makes it possible to realize a light emitting diode device free from the shift of the peak wavelength due to the injection of electric current.
A gallium nitride semiconductor light emitting device according to an embodiment of the present invention comprises a semiconductor substrate, an active layer having a quantum well structure and made of nitride semiconductor containing at least indium and gallium, and a first cladding layer and a second cladding layer for sandwiching the active layer therebetween, and the active layer is composed of two quantum well layers and one barrier layer interposed between the quantum well layers.
When this gallium nitride semiconductor light emitting device is used as a semiconductor laser device, the active layer forms an oscillating section of the semiconductor laser device. Besides, when a driving circuit for injecting an electric current into the semiconductor laser device is provided, a semiconductor laser light source device is realized. Meanwhile, when the gallium nitride semiconductor light emitting device is used as a semiconductor light emitting diode device, the active layer forms a light emitting section of the semiconductor light emitting diode device.
In making the present invention as described above, the present inventor investigated in detail the causes of the aforementioned problems of the conventional devices. As a result, the following was found out.
First, with regard to blue LDs, in the InGaN material to be used for a quantum well layer, both electrons and holes have large effective masses and numerous crystal defects are present, causing the mobility of the electrons and holes to considerably lower, so that the electrons and holes are not distributed uniformly in all the quantum well layers of the multi-quantum-well structure active layer. That is, electrons are injected into only two or so of the quantum well layers on the n-type cladding layer side for electron injection, and holes are injected into only two or so of the quantum well layers on the p-type cladding layer side for hole injection. Accordingly, in the multi-quantum-well structure active layer having five or more quantum well layers, because of a small percentage or rate at which electrons and holes are present in the same quantum well layer, the efficiency of light emission by recombination of electrons and holes lowers, causing the threshold current value of laser oscillation to increase.
Also, because of the low mobility of electrons and holes as shown above, the move of electrons and holes between quantum well layers is slowed down so that electrons and holes cannot be newly injected into the quantum well layers from which electrons and holes have already been disappeared by recombination, and that the electrons and holes that have been injected into quantum well layers close to the cladding layers remain present in the same quantum well layers as they are. Accordingly, even if the injection current is modulated, the densities of electrons and holes present in the quantum well layers are not modulated. This is why injection of a high frequency current does not modulate the optical output power.
In the light of this finding, in the embodiment of the present invention, two quantum well layers are provided in the active layer made of nitride semiconductor containing at least indium and gallium, so that electrons and holes are uniformly distributed in all the quantum well layers. This realizes the improvement of the emission efficiency and hence the reduction of the oscillation threshold current value. Further, because the injection of electrons and holes into the quantum well layers from which electrons and holes have disappeared due to their recombination is effectively achieved, the injection of a high-frequency current successfully modulates the densities of electrons and holes present in the quantum well layers and hence the optical output power.
For making electrons and holes uniformly distributed in all the quantum well layers like this, because too large a layer thickness of a quantum well layer would hinder electrons and holes from being uniformly distributed, each of the quantum well layers preferably has a thickness of 10 nm or less.
Likewise, because too large a layer thickness of the barrier layer would hinder electrons and holes from being uniformly distributed, the barrier layer preferably has a thickness of 10 nm or less.
Meanwhile, with regard to blue LEDs, practically used devices have a tendency that the optical output power comes to be saturated as the current is injected more and more, as shown in FIG. 9. In the conventional blue LEDs, which have only one quantum well active layer, injected electrons and holes are both present in this one quantum well layer, but with the increasing amount of injection, the distribution of injected electrons and holes spreads widely within the momentum space because of the large effective masses of the electrons and holes in InGaN that forms the quantum well layer, with the result that the emission efficiency is lowered. Therefore, with the provision of the two quantum well layers in the multi-quantum-well structure active layer made of a nitride semiconductor containing at least indium and gallium, as in the present invention, injected electrons and holes are divided into the two quantum well layers, by which the densities of electrons and holes present per quantum well layer are reduced. Thus, the distribution of electrons and holes in the momentum space is reduced. As a result of this, the tendency of saturation in the current vs. optical output power characteristic has been mended, and a gallium nitride LED device with high brightness attributable to improved optical output power has been realized.
Further, another investigation and experiment that the present inventor performed proved that with a 4 nm or lower thickness of the barrier layer, even if the quantum well layers are increased in number up to four, results similar to those described above could be obtained in both LDs and LEDs. The quantum well structure active layer of the conventional device described in the foregoing literature, xe2x80x9cApplied Physics Letters, vol. 69, No. 20, p. 3034-3036xe2x80x9d, has three quantum well layers, but because of the large effective masses of electrons and holes of the InGaN material as well as a large thickness of the barrier layer of as much as 8 nm, the wave functions of electrons and holes hardly overlap between the quantum well layers. Therefore, there occur almost no moves of electrons or holes between the quantum well layers, which has caused nonuniform distribution of electrons and holes more noticeably. However, it has been discovered that even if three or four quantum well layers are provided, the wave functions of electrons and holes can be overlapped between the quantum well layers by setting the thickness of barrier layers to 4 nm or less.
It has also been found out that setting the thickness of the barrier layer to 4 nm or less simultaneously solves the problem of peak wavelength shift due to current injection. The cause of such wavelength shift could be considered as follows. That is, in the InGaN material, the electron-hole plasma effect is noticeable because of the large effective masses of electrons and holes, so that energy band ends is largely deformed due to this effect, resulting in an increased shift of the peak emission wavelength due to the current injection. Therefore, it can be concluded that as a result of suppressing the electron-hole plasma effect by reducing the densities of electrons and holes per quantum well layer in such a way that the injected electrons and holes are divided uniformly into the individual quantum well layers as in the present invention, the wavelength shifts due to the current injection have also been reduced.
Further objects, features and advantages of the present invention will be understood from the detailed description of several embodiments thereof which will be given below with reference to the accompanying drawings.